The present invention relates to a method of protecting an object subjected to insert, injection molding by encasing the object in an encapsulating shell prior to subjecting the object to insert, injection molding. More particularly, it relates to encapsulating an object with a heat-shrinkable material prior to subjecting the object to insert, injection molding.
Insert, injection molding involves the insertion of an object into an injection mold and injecting a flowable polymer at high temperature and pressure into the mold to form a molded product of which the inserted object has become an integral part. Certain objects can be damaged when contacted by the high temperature, high pressure polymer as it is injected into the mold. For example, electrical or electronic devices, or combinations thereof in the form of a printed circuit board, are particularly susceptible to damage when subjected to insert, injection molding. Such devices may become instantly inoperative upon contact by molten polymer. Alternatively, thermal shock from direct exposure to molten polymer may produce latent damage, resulting in the premature failure of such parts. In addition, when the injected polymer is above the melt temperature of solder, any solder coming into contact with the polymer will melt. As a consequence, the electrical connection made by the melted solder can become disabled. Moreover, if the solder is only partially melted, the melted portion may flow with the molten polymer and make contact with an adjoining electrical or electronic component, thereby forming a conductive "bridge" between two components which should not be electrically joined.
Although electrical and electronic devices, as well as printed circuit boards, are susceptible to damage when subjected to insert, injection molding, in many instances it is highly desirable to do so nevertheless Insert, injection molding is done in order to provide a protective enclosure around such devices when used in operating environments which may subject the devices to physical abuses, such as jarring or frequent exposure to dirt and moisture. In an automotive vehicle, for instance, electrical devices, electronic devices, and printed circuit boards are often provided with a protective molded enclosure, particularly those which are exposed to automotive fluids, excess vibrations and jarring, or physical contact. Examples include temperature sensors, which are exposed to engine vibrations, continuous contact with oil, and high temperatures, and speed sensors, which are subjected to transmission vibrations, high temperatures, and exposure to transmission fluid.
An additional problem with insert, injection molded products is that the injection molded enclosure can allow liquids or other foreign materials to penetrate the molded enclosure and come into contact with the enclosed object. This can result from cracks in the enclosure as well as through openings intentionally left in the enclosure in order to access the enclosed object. In the case of electrical or electronic components, such liquid contact can result in damage or shorting.
As can be appreciated, means for protecting objects from damage when subjected to insert, injection molding, as well as for preventing contact by liquids which penetrate the molded enclosure, would be highly advantageous. U.S. Pat. No. 3,226,463 to Wallace discloses a dielectric sleeve which is placed around a circuit element as well as the bare wire connections of the circuit element with conductor leads. The circuit element with the sleeve therearound is placed in an injection mold and subjected to heated liquid plastic which is injected into the mold under pressure. However, the dielectric sleeve permits the heated liquid plastic to flow inside of the sleeve such that the heated liquid plastic directly contacts the circuit element and bare wire connections Thus, the circuit element, as well as the connections, is susceptible to thermal damage In addition, any solder used to make the connections is vulnerable to melting. Further, in use, any liquids which penetrate the solidified plastic will directly contact the circuit element and/or the connections.
Accordingly, it is seen that a need exists in the art for a method of protecting an object subjected to insert, injection molding from damage due to contact with the injected polymer, and for protecting the enclosed object from contact from foreign liquids or other materials which penetrate the molded enclosure.